And how is it to be determined which of these powers the people honor and worship?
It is determined by the law which they keep.

If they keep the law of God as given by him, they worship and obey God.

If they keep the law as changed by the papacy, they worship that power.

But further, the prophecy does not say that the little horn should set aside the law of God and give one entirely different.

This would not be to change the law, but simply to give a new one.